1. Field of the Invention
The present invention relates to an organic EL (electroluminescence) light source. Note that an organic EL element is also referred to as an OLED (Organic Light-Emitting Diode).
2. Description of the Related Art
An organic EL light source is superior in response since the time taken from electrical conduction to light emission is extremely short. In addition, the light source has the characteristics in which the response thereof is almost free from variation with temperature and the view angle thereof is close to 180°. Unlike a liquid crystal device that does not emit light by itself, the organic EL light source does not need any backlight and can be made thin and lightweight. For these reasons, the organic EL light source has been a focus of attention in recent years as a device for use in a display unit that is an alternative to a liquid crystal display device and for use in lighting applications.
FIG. 1 is a cross-sectional view illustrating a configuration of commonly-used organic EL light source 9. As illustrated in FIG. 1, luminescent layer 4, which is a principal part of organic EL light source 9, includes organic luminescent element 1 which, when electrified, emits light, transparent electrode 2 having optical transparency, and counter electrode 3 having light reflectivity. In addition, organic luminescent element 1 is disposed between transparent electrode 2 and counter electrode 3 to configure luminescent layer 4. Furthermore, translucent material substrate 5 for fixing luminescent layer 4 is provided on a surface of transparent electrode 2 opposite a surface thereof with which organic luminescent element 1 has contact.
In organic EL light source 9, luminescence takes place in organic luminescent element 1 that has a higher refractive index than the atmosphere. The light thus emitted propagates through transparent electrode 2, translucent material substrate 5, and the atmosphere in this order. The refractive indexes of these mediums generally decrease in the order of organic luminescent element 1, transparent electrode 2, translucent material substrate 5 and the atmosphere, though this depends on the raw materials used for the respective mediums.
When advancing from a high-refractive index medium to a low-refractive index medium, the light may not in some cases pass through a boundary face therebetween, thus causing total reflection. Total reflection occurs if an incidence angle formed by a normal line of the boundary face and incident light is larger than a certain angle (generally referred to as a critical angle). Total reflection follows Snell's law, and the critical angle becomes smaller with the increase of a refractive index difference between two mediums, thus causing the range of incidence angles at which total reflection occurs to become wider.
In organic EL light source 9, the refractive index difference is largest between translucent material substrate 5 and the atmosphere. Consequently, some portion of light emitted by organic luminescent element 1 is confined within translucent material substrate 5 due to total reflection. The efficiency at which light emitted by organic luminescent element 1 is output to the outside (hereinafter referred to as “luminous efficiency”) is said to be only about 20% to 30%.
In addition, a liquid crystal display device or a lighting device, which draws attention as an application of organic EL light source 9, is in many cases viewed by a user from the front side of the device. It is therefore desirable that the front side luminance of the device be high. Accordingly, it has been a challenge to improve luminous efficiency and to enhance the front side luminance in order to effectively utilizing organic EL light source 9.
Hence, JP2004-184792A (hereinafter referred to as Patent Document 1) and JP2004-233957A (hereinafter referred to as Patent Document 2) disclose a method for improving luminous efficiency by use of luminance-enhancing film 14 formed of such transparent resin as illustrated in FIG. 2. FIG. 2 is a cross-sectional view when luminance-enhancing film 14 is fixed onto conventional organic EL light source 9. Luminance-enhancing film 14 is provided on a surface of translucent material substrate 5 opposite a surface thereof with which luminescent layer 4 has contact. The refractive index of luminance-enhancing film 14 is lower than that of translucent material substrate 5 but higher than that of the atmosphere. As a result, a critical angle at a boundary face between translucent material substrate 5 and luminance-enhancing film 14, is larger than a critical angle at a boundary face between translucent material substrate 5 and the atmosphere. Consequently, even light having such an incidence angle as to cause the light to be totally reflected at the boundary face between translucent material substrate 5 and the atmosphere, and therefore, to be not emitted from translucent material substrate 5, can enter from translucent material substrate 5 into luminance-enhancing film 14. Accordingly, a larger amount of light is emitted to the outside through luminance-enhancing film 14.
As illustrated in FIG. 2, luminance-enhancing film 14 includes entrance plane 10 having a planar surface and having contact with translucent material substrate 5 and exit plane 11 having a plurality of convex portions on a surface of luminance-enhancing film 14 opposite entrance plane 10. On the top end of each convex portion in exit plane 11, there are formed top face 12 parallel with entrance plane 10 and inclined plane 13 tilted with respect to entrance plane 10, so as to reduce the size of the area toward the top end.
Accordingly, even light L3, whose incidence angle exceeds a critical angle at top face 12, decreases below the critical angle at inclined plane 13 and can thus be made to transmit through luminance-enhancing film 14 (light L4). As a result, the luminous efficiency of organic EL light source is improved.
However, use of the luminance-enhancing film in order to improve the luminous efficiency of an organic EL light source may add an expense for the luminance-enhancing film and a step of bonding the film to the translucent material substrate, thus increasing the cost of the light source. In addition, there is the possibility that the luminous efficiency of the organic EL light source degrades due to damage to the luminance-enhancing film or the peeling off thereof.
Hence, JP3991862B (hereinafter referred to as Patent Document 3) discloses a technique for improving luminous efficiency without using any luminance-enhancing film. In Patent Document 3, the shape of the luminance-enhancing film in Patent Document 1 or Patent Document 2 is formed in the translucent material substrate to reduce the amount of light to be confined within the translucent material substrate, thereby enhancing luminous efficiency. However, although Patent Document 3 mentions a truncated pyramidal shape, a curved shape, a triangular shape, and the like whereby a light-emitting plane has the convex shape of the translucent material substrate, the document does not show any specific dimensional examples of the shape.
Patent Document 2 discloses the shape of each convex portion of the luminance-enhancing film, specifically the area ratio of the top face of the convex portion to the bottom face of the convex portion, and a range of angles formed by the normal line of the entrance plane and the inclined plane. However, these dimensional data items merely mean a range of shapes of the luminance-enhancing film. Thus, the document does not show any range of shapes when convex portions are provided in the translucent material substrate.